The purpose of this project is to elucidate the structure of phosphofructokinase from muscle, liver and erythrocytes, and to investigate regulation of the enzymes by interaction with other proteins. A possible interaction between fructose-diphosphatase and phosphofructokinase is being studied in order to understand the control mechanism of fructose-6-P and fructose-P2 "futile" cycle. This study is now extended to investigate an interaction with other proteins such as myosin and actin. The enzymatically active forms of phosphofructokinase are now studied by ultracentrifugation method using a Spinco Model E equipped with an optical multichannel analyzer. An investigation will be carried outinto a possible existence of abnormal phosphofructokinase in erythrocytes in patients with phosphofructokinase deficiency disease. A possible regulation of liver phosphofructokinase by free fatty acid, lipids and triglycerides is also studied. An existence of inactive and active forms of phosphofructokinase in liver and the mechanism of the interconversion are investigated.